The present invention relates to a two-component developer, and more particularly to a two-component developer used in an image forming apparatus such as an electrostatic copying apparatus.
In an image forming apparatus such as a copying apparatus, there is widely used a so-called Carlson process comprising the steps of: uniformly charging a photosensitive material with electricity by corona discharge; exposing the charged photosensitive material to light, thereby to form an electrostatic latent image corresponding to a document image; developing the electrostatic latent image by a developer, thereby to form a toner image; transferring the toner image to a medium such as paper; and fixing the toner image transferred to the medium.
As the developer used in the developing step, there is widely used a two-component developer comprising a carrier and toner. The carrier comprises a core material and a coating layer made of a polymer which coats the surface of the core material. The carrier causes the toner to be positively or negatively charged by friction charging. The carrier also causes the toner to be sticked to the surface thereof, and supplies the toner to the surface of the electrostatic latent image.
In electrostatic copying using a conventional two-component developer, however, there occurs a phenomenon which is generally called "carrier scattering" that the carrier together with the toner is adhered or sticked to the surface of an electrostatic latent image. This may produce fine white spots on the resultant copied image. Such white spots are also called blanking.
The following may be presumed as the cause of such carrier scattering.
By the edge effect (edge phenomenon) that the density of the image at the center portion thereof is thinner than at the periphery portion thereof, the potential of the periphery portion of the image is lower in level than the residual potential. Accordingly, the photosensitive drum presents potential difference V.sub.1 between the potential of the black peripheral portion of a black (solid-black) image (1) and the residual potential, as shown in FIG. 1. In an adjacent-line image (2), potential difference V.sub.2 between the potential between adjacent lines, and the residual potential is influenced by the potential of the outer peripheral portions of both adjacent lines. Accordingly, the potential difference V.sub.2 is greater than V.sub.1 (V.sub.2 is approximately equal to 2V.sub.1). In a fine-mesh image (3), potential difference V.sub.3 between the potential of the white portion surrounded by the respective lines and the residual potential becomes greater than the potential difference V.sub.2 of the adjacent-line image (2) (V.sub.3 is greater V.sub.2 which is greater than V.sub.1). On the other hand, a bias voltage having the same polarity as that of the electrostatic latent image is applied to the sleeve of the image forming apparatus. Accordingly, the carrier separated from the sleeve is liable to be adhered to the image peripheral portion based on the principle of inversion phenomenon, thus producing carrier scattering. Such carrier scattering is apt to appear frequently in the order of the solid-black image (1), the adjacent-line image (2) and the mesh image (3), as apparent from the description hereinbefore.